The present invention generally relates to semiconductor memory devices, and more particularly to a semiconductor memory device having an electrically erasable programmable read only memory (EEPROM) function and a dynamic random access memory (DRAM) function. The present invention also provides a method of writing, reading and erasing information in such a semiconductor memory device.
The EEPROM is known as a non-volatile memory which is electrically erasable and programmable. In the EEPROM, a memory cell is made up of at least two transistors, that is, a selection transistor and a storage transistor. For this reason, it is difficult to reduce the area occupied by the memory cells. In addition, the information is read out from the memory cell of the EEPROM by a current detection, thereby making a high-speed read operation difficult.
Therefore, in the conventional EEPROM, there are problems in that it is difficult to improve the integration density by reducing the area occupied by the memory cells, and that it is difficult to read stored information from the memory cell at a high speed.
On the other hand, there is a proposal to use a ferroelectric layer as a dielectric layer of a storage capacitor of the DRAM to realize a non-volatile memory utilizing a hysteresis characteristic of the ferroelectric layer. However, an inexpensive and ferroelectric material having a uniform characteristic is yet to be found, and in addition, it is difficult to insert the process of forming the ferroelectric layer between the existing processes of producing the semiconductor memory device.
Hence, there is a demand to realize a semiconductor memory device which can realize the ROM function by obtaining the hysteresis characteristic without the use of a special material and also realize the RAM function. Consequently, a method of writing, reading and erasing information in such a semiconductor memory device is likewise required.